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Science 17 June 2005:
Vol. 308. no. 5729, pp. 1765 - 1769
DOI: 10.1126/science.1113094

Reports

Spectral Signatures of Hydrated Proton Vibrations in Water Clusters

Jeffrey M. Headrick,1 Eric G. Diken,1 Richard S. Walters,2 Nathan I. Hammer,1 Richard A. Christie,3 Jun Cui,3 Evgeniy M. Myshakin,3 Michael A. Duncan,2* Mark A. Johnson,1* Kenneth D. Jordan3*

The ease with which the pH of water is measured obscures the fact that there is presently no clear molecular description for the hydrated proton. The mid-infrared spectrum of bulk aqueous acid, for example, is too diffuse to establish the roles of the putative Eigen (H3O+) and Zundel (H5O2+) ion cores. To expose the local environment of the excess charge, we report how the vibrational spectrum of protonated water clusters evolves in the size range from 2 to 11 water molecules. Signature bands indicating embedded Eigen or Zundel limiting forms are observed in all of the spectra with the exception of the three- and five-membered clusters. These unique species display bands appearing at intermediate energies, reflecting asymmetric solvation of the core ion. Taken together, the data reveal the pronounced spectral impact of subtle changes in the hydration environment.

1 Sterling Chemistry Laboratory, Yale University, Post Office Box 208107, New Haven, CT 06520, USA.
2 Department of Chemistry, University of Georgia, Athens, GA 30602, USA.
3 Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA.

* To whom correspondence should be addressed. E-mail: maduncan{at}uga.edu (M.A.D.), mark.johnson{at}yale.edu (M.A.J.), jordan{at}pitt.edu (K.D.J.)

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THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Structures and spectral signatures of protonated water networks in bacteriorhodopsin.
G. Mathias and D. Marx (2007)
PNAS 104, 6980-6985
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Sequential Proton Transfer Through Water Bridges in Acid-Base Reactions.
O. F. Mohammed, D. Pines, J. Dreyer, E. Pines, and E. T. J. Nibbering (2005)
Science 310, 83-86
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Science. ISSN 0036-8075 (print), 1095-9203 (online)